Optical fibre having resistance to hydrogen-induced attenuation

ABSTRACT

An optical fiber having resistance to hydrogen-induced attenuation includes a core and cladding including silica. At least one of the core and the cladding includes a dopant capable of not increasing reactivity of the silica with hydrogen. An optical fiber assembly includes a core and cladding including silica. At least one of the core and the cladding includes a dopant capable of changing the refractive index of the fiber core or cladding while not increasing reactivity of the fiber with hydrogen. The optical fiber in some examples further includes a hermetic layer disposed about the cladding. Some implementations include a “getter” layer, which may be an outside part of the fiber cladding been inside the hermetic coating. The “getter” layer includes silica and a dopant increasing reactivity of the layer with hydrogen. The optical fiber assembly optionally includes a sheath disposed about the cladding.

BACKGROUND

1. Field of the Invention

The present invention relates to optical fibers.

2. Description of Related Art

Optical fibers used in harsh environments often degrade over time. Aprimary source of degradation in oilfield applications is attack byhydrogen. The source of hydrogen in such applications is oftencorrosion. The amount of hydrogen generated by corrosion typicallyincreases with temperature. Diffusion of hydrogen into optical fibersalso increases with temperature. Generally, hydrogen diffusion intooptical fibers causes optical signals to attenuate at particularwavelengths.

Typical optical fibers are made of silica (SiO₂) having a core dopedwith germanium. Typical optical fibers are not normally intended for useat temperatures above about 80° C. When these types of optical fibersare exposed to hydrogen, the optical attenuation increases at differentrates, depending upon the wavelength of the optical signal, due tointeractions between hydrogen and the silica of the optical fibers. Themain features of the attenuation spectrum in the infrared region, asshown in FIG. 1, are a steep increase in attenuation at shorterwavelengths, known as “short wavelength edge,” and one or moreabsorption peaks related to hydroxyl (OH) groups generated by thereaction of hydrogen with the silica.

One type of optical fiber assembly known in the art addresses theproblem of hydrogen diffusion by providing a hydrogen retarding layerabout one or more glass layers. The hydrogen retarding layer slows thediffusion of hydrogen into the one or more glass layers. The hydrogenretarding layer, however, is most effective at lower temperatures, suchas those temperatures encountered near the surface of an oil or gas wellor in lower temperature wells, e.g., at temperatures less than about150° C. Using a hydrogen retarding layer on the outside of an opticalfiber, however, is less effective at the higher temperatures found indownhole oil and gas well implementations, where temperatures can reachwell over 300° C.

There continues to be a need for optical fibers that can maintainoptical properties even when exposed to hydrogen at elevatedtemperatures encountered in wellbore applications.

SUMMARY OF THE INVENTION

One aspect of the invention is an optical fiber having resistance tohydrogen-induced attenuation. The optical fiber comprises a coreincluding silica and a cladding including silica disposed about theexterior of the core. At least one of the core and the cladding includesa dopant that changes refractive index of the silica but does notsubstantially increase reactivity of the silica with hydrogen. In someexamples, the optical fiber includes a hermetic layer disposed about theexterior of the cladding. In some examples, the optical fiber includes asheath disposed about the exterior of the cladding.

Additional aspects, features and advantages will be apparent in thewritten description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary graph illustrating a conventional shortwavelength edge and conventional a hydroxyl absorption peak in opticalfibers made using techniques known in the art prior to the presentinvention.

FIG. 2 is a stylized, cross-sectional view of an illustrative example ofan optical fiber assembly according to the present invention.

FIG. 3 is a stylized, cross-sectional view of an illustrative example ofan optical fiber cable according to the present invention.

FIG. 4 is a graph illustrating exemplary optical losses versuswavelength for a phosphorus-doped optical fiber according to the presentinvention both before and after being subjected to hydrogen.

FIG. 5 is a graph illustrating exemplary optical losses versuswavelength for a fluorine-doped optical fiber according to the presentinvention both before and after being subjected to hydrogen.

FIG. 6 is a graph illustrating exemplary optical losses versuswavelength for a nitrogen-doped optical fiber according to the presentinvention both before and after being subjected to hydrogen.

FIG. 7 is a graph illustrating an exemplary index profile for analuminum-doped optical fiber according to the present invention.

FIG. 8 is a graph illustrating exemplary optical losses versuswavelength for a germanium doped fiber and a germanium doped fiber usingvarious amounts of phosphorous as a co-dopant;

FIG. 9 is a graph showing a comparison of test results for variousdopants according to the invention.

DETAILED DESCRIPTION

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedeveloper's specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

The present invention represents an optical fiber particularly suitedfor use in high temperature environments. The optical fiber comprisessilica glass doped with one or more oxidizer elements, which do notsignificantly modify the silica glass structure and do not formsignificant amounts of precursors to react with hydrogen. The opticalfiber may be incorporated into an optical fiber assembly, which caninclude a hermetic coating applied about the optical fiber. The opticalfiber assembly can further include a protective sheath disposed aboutthe optical fiber or disposed about the hermetic coating, if present.Other conventional elements of optical fiber assemblies may be includedin the present optical fiber assembly.

FIG. 2 depicts an illustrative embodiment of an optical fiber assembly201 according to the present invention. In the illustrated embodiment,optical fiber assembly 201 comprises an optical fiber 203 including acore 205 and a cladding 207, a hermetic layer 209 disposed about opticalfiber 203, and a sheath 211 disposed about hermetic layer 209.Alternatively, however, hermetic layer 209 or sheath 211 may be omitted.If hermetic layer 209 is omitted, sheath 211 is disposed about opticalfiber 203. It should also be noted that the present inventioncontemplates one or more optical fibers 203, omitting both hermeticlayer 209 and sheath 211.

Core 205 of optical fiber 203 comprises silica glass may be doped withone or more elements, which do not significantly modify the silica glassstructure and do not form significant amounts of precursors to reactwith hydrogen. The doping elements are capable of changing therefractive index of the fiber core or cladding but at the same time notsubstantially increasing reactivity of the fiber 203 with hydrogen.Examples of the elements or “dopants” include, but are not limited to,nitrogen, fluorine, phosphorus, and aluminum.

In one embodiment, optical fiber 203 is constructed by forming agenerally elongated cylindrical, optical waveguide structure, also knownas a “preform” or a “blank.” The optical waveguide structure ispreferably formed using a chemical vapor deposition process, which maybe plasma-assisted. In one such process, oxygen is bubbled throughsolutions comprising the one or more dopant elements. The resultingvapors are then conducted to an internal cavity of a silica or quartztube, which vapors ultimately form a cladding 207, while the tube isrotated generally about its longitudinal. As the tube is rotated, thetube is locally heated to a high temperature sufficient to cause the oneor more dopant elements to react with oxygen, thus forming correspondingone or more oxides. The oxides are deposited on and fused to the insideof the tube, or are deposited on and fused to previously depositedoxide. The process is continued until a solid optical waveguidestructure is formed.

It should be noted that the present invention contemplates an opticalfiber that varies in composition in a radial direction from a central,longitudinal axis of the fiber. For example, in one embodiment, core 205of optical fiber 203 is aluminum-doped in a central portion 213(indicated by a dashed line) thereof. While central portion 213 of core205 is depicted in FIG. 2 as having a particular size and a particularsize with respect to core 205, the scope of the present invention is notso limited. Rather, central portion 213 may exhibit any suitable sizeand any suitable size with respect to core 205. In some examples, thecore 205 and/or the cladding 207 may be made using silica doped with oneor more dopants as will be described in more detail below. An exteriorlayer to the cladding 207 may be doped only with germanium in amountstypical for such doping as is known in the art. Having a germanium-dopedexterior layer may provide the optical fiber with a reactive “getter”layer to reactively absorb hydrogen and reduce its diffusion into thecladding 207 and the core 205.

The optical waveguide structure is then drawn into optical fiber 203 ofthe present invention. During the drawing operation, the hermetic layer209 may be applied to optical fiber 203 to further protect againsthydrogen diffusion into core 205. Alternatively, hermetic layer 209 maybe applied to optical fiber 203 after the drawing process. Preferably,hermetic layer 209 comprises carbon or a metallic material.

Referring to FIG. 3, optical fiber 203, with or without hermetic layer209, may be bundled with one or more other optical fibers 203 in anoptical fiber cable or assembly 301. In FIG. 3, an optical fiberassembly 303 comprises optical fiber 203 and hermetic layer 209. Itshould be noted that optical fibers 203 of optical fiber cable 301 mayhave arrangements that are different than the arrangement shown in FIG.3. Moreover, in an alternative embodiment, one or more optical fibers203 of optical fiber cable 301 may be replaced with conventional opticalfibers or other conductors, such as electrical conductors, such that atleast one optical fiber 203 is present in optical fiber cable 301.Optical fiber cable 301 preferably includes a sheath 305 formed aboutthe one or more optical fibers 203 to protect the one or more opticalfibers 203 from damage. A filler 307 may be disposed between sheath 305and optical fiber assemblies 303.

The graphs of FIGS. 4-9 provide the results of a series of testsconducted involving various embodiments of optical fiber 203. FIG. 4graphically shows results of tests performed on a phosphorus-dopedoptical fiber 203. The dashed line in the graph illustrates a relativelylow level of initial losses at wavelengths within a range of about 800nm to about 1600 nm prior to the introduction of hydrogen. The solidline represents the attenuation of optical signals at wavelengths withina range of about 800 nm to about 1600 nm after subjecting optical fiber203 to hydrogen at a pressure of about 50 atmospheres and at atemperature of about 300° C. for about 6 hours, followed by hydrogenout-diffusion. While losses are high at wavelengths above about 1350 nm,no short wavelength edge is exhibited. Such an embodiment of opticalfiber 203 is particularly well-suited for many downhole, oilfield,distributed temperature sensing applications, which operate atwavelengths of about 1060 nm.

FIG. 5 relates to tests performed on a fluorine-doped optical fiber 203.The dashed line in the graph depicts only slight attenuation of opticalsignals at wavelengths within a range of about 800 nm to about 1600 nmdue to OH interactions prior to the introduction of hydrogen. The solidline represents the attenuation of optical signals at wavelengths withina range of about 800 nm to about 1600 nm after subjecting optical fiber203 to hydrogen at a pressure of about 50 atmospheres and at atemperature of about 300° C. for about 6 hours, followed by hydrogenout-diffusion. Only OH-related attenuation peaks grew, with no formationof a short wavelength edge. It was also discovered that fluorine-dopedoptical fiber 203 is less sensitive to hydrogen at elevatedtemperatures. As noted above, such an embodiment of optical fiber 203 isparticularly well-suited for many downhole, oilfield, distributedtemperature sensing applications, which operate at wavelengths of about1060 nm.

FIG. 6 relates to tests performed on a nitrogen-doped optical fiber 203.The dashed line in the graph depicts only slight attenuation of opticalsignals at wavelengths within a range of about 800 nm to about 1600 nmdue to OH interactions prior to the introduction of hydrogen. The solidline represents the attenuation of optical signals at wavelengths withina range of about 800 nm to about 1600 nm after subjecting optical fiber203 to hydrogen at a pressure of about 50 atmospheres and at atemperature of about 300° C. for about 6 hours, followed by hydrogenout-diffusion. Only OH-related attenuation peaks grew, with no formationof a short wavelength edge. It was also discovered that, as withfluorine-doped optical fiber 203, nitrogen-doped optical fiber 203 isless sensitive to hydrogen at elevated temperatures. As noted above,such an embodiment of optical fiber 203 is particularly well-suited formany downhole, oilfield, distributed temperature sensing applications,which operate at wavelengths of about 1060 nm.

FIG. 7 graphically shows tests performed on an aluminum-doped opticalfiber (203 in FIG. 2). The dashed line in the graph depicts only slightattenuation of optical signals at wavelengths within a range of about800 nm to about 1600 nm due to OH interactions prior to the introductionof hydrogen. The solid line represents the attenuation of opticalsignals at wavelengths within a range of about 800 nm to about 1600 nmafter subjecting the optical fiber to hydrogen at a pressure of about 50atmospheres and at a temperature of about 300° C. for about 6 hours,followed by hydrogen out-diffusion. Only the OH-related attenuationpeaks increased, and the optical fiber showed essentially no formationof a short wavelength edge. It was also discovered that fluorine-dopedoptical fiber is less sensitive to hydrogen at elevated temperatures. Asnoted above, such an embodiment of optical fiber 203 is particularlywell-suited for many downhole, oilfield applications such as distributedtemperature sensors, which typically operate at wavelengths of about1060 nm.

FIG. 8 graphically shows results of relates performed on agermanium-doped optical fiber co-doped with small amounts of phosphorus.The various curves in the graph represent the attenuation of opticalsignals at wavelengths within a range of about 900 nm to about 1600 nmafter subjecting optical fibers 203 to hydrogen at a pressure of about 1atmosphere and at a temperature of about 300° C. for about 6 hours,followed by hydrogen out-diffusion. One curve represents the responsefor a germanium-only doped fiber. Another curve represents the responsefor a germanium plus 0.3% phosphorus doped fiber. The final curverepresents the response for germanium plus 0.9% phosphorous doped fiber.In the fibers with phosphorus co-doping, OH-related attenuation peaksincreased, however there was significantly smaller formation of a shortwavelength edge than in germanium-doped, phosphorus-free fibers. It wasalso discovered that germanium-doped optical fiber 203 with phosphorusco-doping is less sensitive to hydrogen at elevated temperatures. Asnoted above, such an example of optical fiber 203 is particularlywell-suited for many downhole (in wellbore), oilfield applications,which operate at wavelengths of about 1060 nm.

Aluminum-doped optical fiber 203 was then subjected to hydrogen at apressure of about 1 atmosphere and at a temperature of about 300° C. forabout 130 hours, followed by an increase in hydrogen pressure to about40 atmospheres for an additional time of about 55 hours to acceleratethe test. FIG. 9 depicts the resulting optical attenuations,recalculated for 1 atmosphere of hydrogen pressure. The OH-induced peakat about 1380 nm increased over time. No short wavelength edge, however,was induced in aluminum-doped optical fiber 203. Moreover, regions ofthe measured spectrum other than at the OH-induced peak exhibitedsignificantly lower attenuation values than conventional germanium- orgermanium+phosphorus-doped optical fibers.

Thus, optical fiber 203 inhibits the development of short wavelengthedges and induced attenuation peaks when optical fiber 203 is subjectedto hydrogen. In one embodiment, optical fiber 203 comprises phosphorusdoping to inhibit increases in short wavelength edge attenuation whenoptical fiber 203 is subjected to hydrogen. In another embodiment,optical fiber 203 comprises phosphorus, co-doped with another element,such as fluorine, germanium, nitrogen, or aluminum, to inhibit increasesin short wavelength edge attenuation when optical fiber 203 is subjectedto hydrogen. In yet another embodiment, optical fiber 203 comprisesfluorine doping to inhibit attenuation increases when optical fiber 203is subjected to hydrogen. In another embodiment, optical fiber 203comprises fluorine, co-doped with another element, such as phosphorus,germanium, nitrogen, or aluminum, to inhibit increases in attenuationwhen optical fiber 203 is subjected to hydrogen.

In yet another embodiment, optical fiber 203 comprises nitrogen. Inanother embodiment, optical fiber 203 comprises aluminum.

A comparison of results of testing various dopants used in makingoptical fibers and subjecting the fibers to hydrogen is shown in graphicform in FIG. 9. The various curves in FIG. 9 represent opticalattenuation after hydrogen exposure to silica fibers doped with thevarious elements shown. In comparison with germanium-only doped fibers,all of the tested dopants provided the optical fiber with substantiallyreduced attenuation due to hydrogen diffusion in a wavelength range ofabout 800 to 1200 nm.

The effective amount of any particular doping element may be differentfor each element. For nitrogen, fluorine, phosphorous and aluminum, forexample, the effective amount must be enough to form the appropriaterefractive index profile in the optical fiber. For example silica may bedoped with 0.4 at % nitrogen to 4 at % nitrogen to form the necessaryrefractive index profile depending on the particular application for theoptical fiber. For other dopants, the effective amount may be different.

It will be appreciated by those skilled in the art that the variousdopants are used to modify the refractive index of the silica used tomake the core and the cladding, such that the optical fiber can act as awaveguide. In any example of an optical fiber, therefore, the elementused as a dopant, the amount of the dopant and its inclusion into eitherthe cladding and/or the core should be selected to provide theappropriate refractive index to the core and to the cladding such thatthe optical fiber can act as an optical waveguide. Using dopants assuggested herein may reduce effects of hydrogen on the optical fiber.

The particular examples described above are illustrative only, as theinvention may be modified and practiced in different but equivalentmanners apparent to those skilled in the art having the benefit of theteachings herein. Furthermore, no limitations are intended with respectto the details of construction or design herein shown, other than asdescribed in the claims below. It is therefore evident that theparticular examples described above may be altered or modified and allsuch variations are considered within the scope of the invention.Accordingly, the scope of what has been invented shall be defined onlyby the appended claims.

1. An optical fiber having resistance to hydrogen-induced attenuation,comprising: a core including silica; and a cladding including silica, atleast one of the core and the cladding including a dopant capable ofchanging the refractive index of at least one of the core and thecladding and not substantially increasing reactivity of the silica withhydrogen.
 2. The optical fiber according to claim 1, wherein the dopantcomprises an element selected from the group consisting of nitrogen,fluorine, phosphorus, and aluminum.
 3. The optical fiber according toclaim 1, wherein the dopant is capable of inhibiting generation of ashort wavelength edge within a range of wavelengths from about 800nanometers (nm) to about 1600 nm when the core is subjected to hydrogen.4. The optical fiber according to claim 1, wherein the dopant is capableof inhibiting attenuation, other than hydroxyl-induced attenuation,within a range of wavelengths from about 800 nm to about 1600 nanometerswhen the core is subjected to hydrogen.
 5. The optical fiber accordingto claim 1, wherein the core further comprises a central portion dopedwith aluminum.
 6. The optical fiber according to claim 1, wherein thedopant comprises phosphorus co-doped with an element selected from thegroup consisting of fluorine, germanium, nitrogen, and aluminum.
 7. Theoptical fiber according to claim 1, wherein the dopant comprisesfluorine.
 8. The optical fiber according to claim 1, wherein the dopantcomprises nitrogen.
 9. The optical fiber according to claim 1, whereinthe core includes germanium as a dopant co-doped with phosphorus. 10.The optical fiber according to claim 1, wherein the dopant comprisesaluminum.
 11. The optical fiber according to claim 1, wherein theoptical fiber is formed using a chemical vapor deposition process. 12.The optical fiber according to claim 11, wherein the chemical vapordeposition process is plasma-assisted.
 13. The optical fiber accordingto claim 1, further comprising a silica layer disposed outside thecladding, the silica layer doped with a material causing higherreactivity to hydrogen than the dopant.
 14. The optical fiber accordingto claim 13, wherein the material comprises germanium.
 15. An opticalfiber assembly, comprising: a core including silica; and a claddingincluding silica, at least one of the core and the cladding including adopant capable of changing the refractive index of the fiber core orcladding and not substantially increasing reactivity of the fiber withhydrogen; and a hermetic layer disposed about the cladding.
 16. Theoptical fiber assembly according to claim 15, wherein the dopantcomprises: an element selected from the group consisting of nitrogen,fluorine, phosphorus, and aluminum.
 17. The optical fiber assemblyaccording to claim 15, further comprising: a sheath disposed outside thefiber cladding and outside the hermetic layer.
 18. The optical fiberassembly according to claim 17, further comprising: a filler disposedbetween the sheath and the hermetic layer.
 19. The optical fiberassembly according to claim 15, wherein the core further comprises acentral portion doped with aluminum.
 20. The optical fiber assembly,according to claim 15, wherein the fiber includes germanium as a dopant.21. The optical fiber assembly according to claim 15, further comprisinga silica layer disposed in an outside part of the fiber cladding, thesilica layer doped with a material causing higher reactivity to hydrogenthan the reactivity of pure silica.
 22. The optical fiber assembly ofclaim 21, wherein the material comprises germanium.
 23. An optical fiberassembly, comprising: a core including silica; a cladding disposed aboutthe core, the cladding including silica, at least one of the core andthe cladding including a dopant capable of changing the refractive indexof the silica and not substantially increasing reactivity of hydrogenwith the silica; and a sheath disposed about the cladding.
 24. Theoptical fiber assembly according to claim 23, wherein the dopantcomprises: an element selected from the group consisting of nitrogen,fluorine, phosphorus, and aluminum.
 25. The optical fiber assemblyaccording to claim 23, further comprising: a second core includingsilica; and a second cladding disposed about the second core; whereinthe sheath is disposed about the cladding and the second cladding. 26.The optical fiber assembly according to claim 25, wherein the secondcladding includes a dopant capable of changing refractive index of thesilica in the second cladding while not increasing reactivity of thesilica with hydrogen.
 27. The optical fiber assembly according to claim25, further comprising: a hermetic layer disposed about the secondcladding.
 28. The optical fiber according to claim 25, furthercomprising a silica layer disposed outside the cladding, the silicalayer doped with a material causing higher reactivity to hydrogen thanthe dopant.
 29. The optical fiber according to claim 28, wherein thematerial comprises germanium.